It is often necessary to effect a fluid tight seal between two components, and in particular between a rotating shaft and a stationary housing. One type of seal that has been used, especially within the gas turbine engine field, is a brush seal. Such seals have been used within gas turbine engines to seal a gap between a rotating shaft, typically rotating at high speed, and a stationary housing where regions of higher pressure and lower pressure air, which may also be at an elevated temperature, respectively exists on each side of the seal.
Brush seals comprise a plurality of densely packed bristles sandwiched between a front plate and a backing plate. The bristles, and front and backing plate, are all attached to either of the stationary housing or the rotating shaft with the bristles extending across the gap between the shaft and housing to a sealing surface on the other of the shaft or housing. The bristles extend further than the backing plate with the free ends of the bristles generally just touching the sealing surface, or being in close proximity to it. The bristles thereby present a physical barrier to the fluid so sealing the gap between the housing and shaft.
The backing plate provides axial support to the bristles and also, since it is a solid member, partially obstructs the gap between the housing and the shaft. Consequently the backing plate is generally arranged to extend across the majority of the gap between the housing and shaft with a clearance being provided between the free end of the backing plate and the sealing surface to prevent interference between the backing plate and the shaft or housing. This clearance being sealed by the bristles that extend beyond the end of the backing plate and which are compliant so as to flex in response to the shaft movement.
Differential thermal growth, centrifugal growth of the shaft, engine manoeuvring, and build tolerances can all reduce the clearance between the backing plate and the seal surface. In the worst case in which the clearance is reduced to zero the backing plate contacts the seal surface causing unacceptable irreversible damage to the backing plate and seal surface. Consequently a sufficiently large clearance is provided between the backing plate and the sealing surface to accommodate the anticipated reduction in the clearance during operation. Unfortunately the provision of such a large clearance reduces the seal performance. Additionally due to the large clearance required the bristles can bend under the backing plate further reducing the seal performance and causing damage to the bristles.
It is therefore desirable to provide a seal arrangement which addresses the above mentioned problems in which the clearance between the backing ring and the sealing surface is minimised whilst ensuring that excessive interference between the backing plate and the sealing surface does no occur. It is also desirable to provide the improvements to such seals generally.